Method for producing high strength, high ductility steel strip

ABSTRACT

A method is provided for the manufacture of steel steel strip having high strength and high ductility. The method includes hot rolling steel consisting essentially of in weight percent C 0.05/0.20, Mn 3.0/8.0, Si less than 0.5, Al less than 0.1, the balance Fe and inevitable impurities at a minimum finishing temperature of Ar1+50 C, cooling the hot rolled steel strip at a rate sufficient to ensure that the microstructure of the strip consists of greater than 50 percent by volume of martensite, and then annealing the steel strip at a temperature within the range of Ac1 to Ac1+50° C. for a minimum time at temperature of one hour. The steel has a microstructure after annealing that consists essentially of ferrite and retained austenite.

FIELD OF THE INVENTION

The present invention relates to a method for producing high strength,high ductility steel strip, and particularly to a method for producingsteel strip containing 0.05/0.20% carbon, 3 to 8% manganese, less than0.5% silicon, less than 0.1 aluminum, balance iron and incidentalimpurities by hot rolling, cooling to form a microstructure of which amajor portion consists of martensite, and box annealing in theintercritical temperature range to form ferrite and significant amountsof austenite that is retained upon cooling to room temperature.

BACKGROUND OF THE INVENTION

Customers for automotive steel sheet and strip are interested in thedevelopment of steels that possess higher levels of strength than thatof currently available steels, to enable the reduction of steelthickness for automotive applications. Higher strength steels enable theautomotive designer to reduce vehicle weight while maintainingstructural integrity. However, conventional high strength steels lacksufficient ductility to produce complex parts.

This has led to the development of new types of high strength steel thathave greater ductility than conventional high strength steels. Forexample, dual phase steels have been developed that have amicrostructure consisting of ferrite and about 10 to 30 percentmartensite. The dual phase steels typically have ultimate tensilestrength levels ranging from 600 MPa to 1000 MPa with total elongationranging from 25 to 30% at the 600 MPa strength level and about 15% atthe 1000 MPa strength level. Another type of high strength steel thathas been developed is called transformation induced plasticity, or TRIPsteel, that has a ferrite and austenite microstructure. The austenite inthese steels transforms to martensite during deformation or forming bythe customer, thus leading to the name TRIP steel. Currently availableTRIP steels have ultimate tensile strength levels ranging from 600 to800 MPa with total elongation of about 30% at the 600 MPa strength leveland about 28% at the 800 MPa strength level. Of particular interest toautomotive customers are high strength steels having ultimate tensilestrengths in the range of 800 to 1000 MPa with total elongationscomparable or exceeding the 28 to 30% of currently available TRIPsteels.

U.S. Pat. No. 4,437,902 to Pickens et at., discloses a dual phase steelcontaining less than 0.2% carbon, less than 2% manganese and at least0.4% copper and 0.6% nickel that is batch annealed in the intercriticaltemperature range after either hot or cold rolling and slow cooled toproduce a steel having an ultimate tensile strength of at least 80 ksi(551 MPa) and at least 18% total elongation. The disclosed steels areprimarily used as skelp for the production of steel tubing, however,Alloy A of the disclosure is cited as suitable for automotiveapplications. The disclosed steels have a dual phase microstructureconsisting of martensite dispersed in a ferrite matrix. U.S. Pat. No.4,047,979 to Grange et al., assigned to the assignee of the presentinvention, discloses a steel plate product containing about 2 to 6percent manganese that is produced by hot rolling and annealing in theintercritical temperature range to provide high strength and enhancedtoughness at low temperatures as measured by CVN impact energy. JapanesePatent Application Hei 5-311323 discloses steel strip having highstrength and elongation having a composition in weight percent of C0.10-0.20, Si 0.80-1.60, Mn 3.0-6.0, Al 0.5 or less and the rest ironand unavoidable impurities. The steel is hot rolled and coiled at400-600 C and then box annealed in the intercritical phase region of Ac1to Ac1+50 C for 1 to 20 hours and furnace cooled. The steel has amicrostructure composed of 10% by volume of residual austenite, 40 to75% ferrite, and 5 to 40% bainite. The steel has a tensile strength of780 N/mm2 or higher and 25 to 33% elongation. One disadvantage of thisreference steel is that the relatively high Si content causesunfavorable oxidation of the strip surface during hot rolling andcoiling. Removal of the oxidized surface during hot rolling and in asubsequent pickling operation is difficult and requires extra processingtime. Failure to remove the oxidized strip surface results in rejectionof the strip for automotive applications due to undesirable surfaceappearance or unremoved scale.

Under the current state of the art as described above, it is desirableto provide a steel strip suitable for automotive applications that hasan ultimate tensile strength level of at least 800 MPa, with totalelongations exceeding 25 percent, and that is not subject to excessiveoxidation of the strip surface during hot rolling.

DISCLOSURE OF THE INVENTION

According to the present invention a method for the manufacture of steelstrip having high strength and high ductility includes hot rolling steelconsisting essentially of in weight percent C 0.05/0.20, Mn 3.0/8.0, Siless than 0.5, Al less than 0.1, the balance Fe and inevitableimpurities at a minimum finishing temperature of Ar1+50 C, cooling thehot rolled steel strip at a rate sufficient to ensure that themicrostructure of the strip consists of greater than 50 percent byvolume of martensite, and then annealing the steel strip at atemperature within the range of Ac1 to Ac1+50° C. for a minimum time attemperature of one hour. The steel has a microstructure after annealingthat consists essentially of ferrite and retained austenite.

In an alternate embodiment the range of Mn is 2.0/8.0% and the Al iswithin the range of 0.1/2.0% max.

In a preferred form of the invention, the hot rolled steel strip iscooled at a rate sufficient to ensure that the microstructure of thestrip consists of greater than 90 percent by volume of martensite. Inanother preferred form of the invention, the annealing cycle time iscontrolled so that the maximum difference in temperature between anouter lap of the coil and an inner lap of the coil is less than 28° C.(50° F.) during the anneal cycle.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of the product of ultimate tensile strength in MPatimes the percent total elongation at various peak annealingtemperatures for Steel A coiled at three different hot roll coilingtemperatures.

FIG. 2 is a plot of the product of ultimate tensile strength in MPatimes the percent total elongation at various peak annealingtemperatures for Steel B coiled at three different hot roll coilingtemperatures.

FIG. 3 is a plot of the product of ultimate tensile strength in MPatimes the percent total elongation at various peak annealingtemperatures for Steel C coiled at three different hot roll coilingtemperatures.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Tests were conducted in the laboratory to determine whether a highstrength, high ductility steel could be produced utilizing conventionalhot rolling practices and box annealing techniques. Table 1 belowpresents the composition of the steel materials studied in theselaboratory tests. TABLE 1 Steel C Mn P S Si Cu Ni Cr Mo Al N A 0.1005.18 .015 .008 .12 .03 .03 .04 .02 .03 .009 B 0.095 5.80 .013 .008 .13.04 .04 .04 .02 .03 .008 C 0.099 7.09 .015 .008 .13 .03 .03 .04 .01 .03.008

Laboratory ingots of the above compositions were reheated to 1260° C.(2300° F.) and rolled from 178 mm (7) inch thickness to an intermediateslab thickness of 51 mm (2 inches). The slabs were reheated again to thesame temperature and hot rolled in seven passes to 4 mm (0.16 inch)thickness to simulate a conventional hot rolling process. The hot rolledstrips were then placed in a programmable furnace which was set to coolat 28° C./hour (50° F./hour) from the prescribed “coiling temperature”to simulate coiling on a commercial hot strip mill. Three differentfurnace temperatures were used so as to simulate three different coilingtemperatures, namely, 538° C. (1000° F.), 593° C. (110° F.), and 649° C.(1200° F.). After cooling to room temperature, some of the strips wereexamined to determine the microstructure in the “as hot rolledcondition”. This examination revealed that strips had a fullymartensitic microstructure after hot rolling and cooling as justdescribed. The strips were then annealed in a furnace simulatingcommercial box anneal thermal cycles at one of the following aim peaktemperatures 510° C. (950° F.), 538° C. (1000° F.), 566° C. (1050° F.),593°C. (1100° F.), 621° C. (1150° F.), and 649° C. (1200° F.). Theannealed strips were tested to determine mechanical properties and theamount of retained austenite was determined in the microstructure. Theresults for each steel composition are provided in the tables below.

Table 2 below presents the results for Steel A at the three simulatedcoiling temperatures and six peak annealing temperatures along with anas hot rolled sample that was not annealed after hot rolling. TABLE 2Peak % UTS × Coiling Ann. T YS UTS Uniform Tot. YS/TS Retained Tot. T °C. ° C. MPa MPa El. % El. % Ratio Austenite El. 538 N/A 885.0 1254.3 4.19.7 0.7 N/A 12,167 538 510 852.2 884.3 5.6 13.4 1.0 0.2 11,850 538 538808.4 852.6 7.7 16.5 0.9 1.0 14,068 538 566 815.0 853.6 6.7 15.9 1.0 0.513,572 538 593 643.0 750.2 12.2 26.5 0.9 9.0 19,880 538 621 647.8 750.514.4 27.6 0.9 12.1 20,714 538 649 610.9 897.4 26.7 33.8 0.7 24.8 30,332593 N/A 788.8 944.3 4.6 10.0 0.8 N/A  9,443 593 510 849.8 894.3 7.7 16.41.0 0.7 14,667 593 538 782.9 838.4 7.3 16.9 0.9 2.3 14,169 593 566 795.0849.5 7.1 15.3 0.9 0.6 12,997 593 593 649.9 756.4 10.2 20.7 0.9 10.715,657 593 621 675.0 771.9 12.8 24.9 0.9 14.3 19,220 593 649 629.5 886.424.7 30.0 0.7 15.6 26,592 593 654 589.5 772.2 30.3 39.7 0.8 ND* 30,656593 677 359.9 1012.5 16.7 20.9 0.4 ND* 21,161 649 N/A 809.6 1155.2 4.19.2 0.7 ND* 10,628 649 510 832.6 885.3 7.2 15.2 0.9 0.9 13,457 649 538778.8 828.8 7.1 16.3 0.9 1.5 13,509 649 566 774.0 819.5 7.1 16.4 0.9 0.913,440 649 593 651.6 751.6 12.6 25.1 0.9 7.1 18,865 649 621 659.2 750.214.0 27.0 0.9 8.2 20,255 649 649 567.1 897.4 24.3 31.3 0.6 9.0 28,089

Table 3 below presents the results for Steel B at the three simulatedcoiling temperatures and six peak annealing temperatures along with anas hot rolled sample that was not annealed after hot rolling. TABLE 3Peak % Coiling Ann. T YS UTS Uniform Tot. YS/TS Retained UTS × T ° C. °C. MPa MPa El. % El. % Ratio Austenite Tot. El. 538 N/A 858.8 1222.2 3.78.4 0.7 N/A 10,266 538 510 854.9 916.6 7.4 14.7 0.9 1.0 13,474 538 538808.0 877.4 9.2 18.8 0.9 5.2 16,495 538 566 833.8 889.7 8.0 16.7 0.9 6.114,858 538 593 707.8 812.7 13.1 26.8 0.9 15.1 21,780 538 621 687.1 797.215.2 30.0 0.9 14.0 23,916 538 649 596.2 953.3 28.2 36.3 0.6 17.7 34,605593 N/A 775.8 953.6 5.6 11.8 0.8 N/A 11,252 593 510 882.5 918.4 6.1 15.31.0 0.8 14,051 593 538 784.7 859.0 8.3 17.4 0.9 3.0 14,947 593 566 793.1862.4 8.9 17.9 0.9 3.7 15,437 593 593 715.8 819.2 11.4 24.8 0.9 8.620,316 593 621 743.8 830.3 12.2 24.4 0.9 11.3 20,259 593 649 606.0 912.029.3 37.1 0.7 13.5 33,835 593 654 624.7 841.5 31.2 39.5 0.7 ND* 33,239593 677 365.4 1117.0 15.9 19.4 0.3 ND* 21,670 649 N/A 881.9 1315.5 4.110.4 0.7 ND* 13,681 649 510 858.3 922.9 7.5 15.5 0.9 1.0 14,305 649 538785.5 865.8 8.4 18.9 0.9 5.5 16,364 649 566 792.8 864.2 8.5 19.3 0.9 5.216,679 649 593 698.0 813.4 12.0 25.3 0.9 14.3 20,579 649 621 718.1 819.512.0 24.0 0.9 14.7 19,668 649 649 615.3 932.0 28.0 37.2 0.7 18.4 34,670

Table 4 below presents the results for Steel C at the three simulatedcoiling temperatures and six peak annealing temperatures along with anas hot rolled sample that was not annealed after hot rolling. TABLE 4Peak % Coiling Ann. T YS UTS Uniform Tot. YS/TS Retained UTS × T ° C. °C. MPa MPa El. % El. % Ratio Austenite Tot. El. 538 N/A 866.3 1412.6 4.710.3 0.6 N/A 14,550 538 510 921.7 1013.7 8.3 16.7 0.9 2.7 16,929 538 538884.7 991.9 8.5 16.3 0.9 8.9 16,168 538 566 871.9 975.2 9.4 17.1 0.9 8.716,675 538 593 764.4 915.4 18.7 28.6 0.8 22.5  26180 538 621 817.8 942.118.6 27.8 0.9 19.8 26,190 538 649 638.1 1082.9 26.6 28.7 0.6 12.7 31,079593 N/A 771.2 978.3 8.2 15.6 0.8 N/A 15,261 593 510 933.7 982.6 7.8 16.01.0 2.9 15,721 593 538 846.9 967.4 9.5 18.2 0.9 9.2 17,607 593 566 871.1938.6 9.9 19.3 0.9 10.0 18,115 593 593 780.4 887.6 15.9 28.5 0.9 17.425,297 593 621 810.1 904.2 17.4 30.3 0.9 12.1 27,397 593 649 649.21056.5 25.0 27.2 0.6 9.7 28,737 593 654 706.0 949.8 25.6 30.2 0.7 ND*28,684 593 677 388.5 979.1 3.5 3.5 0.4 ND*  3,427 649 N/A 841.3 142546.0 11.0 0.6 ND* 15,679 649 510 937.5 1028.7 8.0 16.7 0.9 2.3 17,179 649538 822.1 967.1 9.5 19.1 0.9 9.1 18,472 649 566 838.2 967.1 10.0 19.60.9 8.6 18,955 649 593 753.5 917.8 16.8 27.7 0.8 12.8 25,423 649 621762.5 917.8 17.8 28.9 0.8 14.2 26,524 649 649 659.7 1074.1 27.6 33.6 0.619.6 36,090

The results of the laboratory tests show that it is possible through theselection of hot roll finish and coiling temperature, and box annealtemperature to obtain exceptionally high ultimate tensile strength andtotal elongation in a plain low carbon steel composition containingmanganese and having low levels of silicon. An important feature is thatthe rate of cooling after finish hot rolling and the coiling temperatureare sufficient to obtain a microstructure after hot rolling thatconsists of at least 50% martensite by volume. Preferably, themicrostructure after finish hot rolling and coiling consists of at least90% martensite and most preferably 100% martensite by volume. For thecomposition of the steels shown in Table 1 the coiling temperature mayrange from 538 to 649° C. The results are more sensitive to boxannealing temperature, with excellent tensile strength and totalelongation being obtained at box annealing temperature within the rangeof 593 to 654° C. The total elongation seems to drop to lower values atbox anneal temperatures of 677° C.

Based on the results of these tests, a steel strip product consistingessentially in weight percent of C 0.05/0.20, Mn 3.0/8.0, Si less than0.5, Al less than 0.1, the balance Fe and incidental impurities, can beproduced through the selection of hot roll finish and coilingtemperature and box anneal temperature so as to have tensile strengthgreater than 800 MPa and total elongation greater than 25%. The steelcan achieve a combination of ultimate tensile strength in MPa multipliedby total elongation percent of 25,000 or more. These representexceptional properties for a plain carbon steel that is suitable forautomotive and other critical applications where exceptionally cleansurface is required. To achieve various strength levels the combinationof carbon and manganese content may be selected based on desiredultimate tensile strength. For a given carbon level increasing themanganese content results in increased ultimate tensile strength andcauses only a slight decrease in total elongation. In one preferredembodiment, a steel strip product having an ultimate tensile strength ofgreater than 800 MPa and total elongation of at least 35% can beproduced from strip consisting essentially in weight percent of C 0.09/0.11, Mn 5.7/6.0, Si less than 0.5, Al less than 0.1, balance Fe andincidental impurities. To achieve these properties the hot rollfinishing temperature should be within the range of 871° C./982° C.(1600° F./1800° F.), with a coiling temperature within the range of 566°C./621° C. (1050° F./1150° F.), and a box annealing temperature withinthe range of 635° C./663° C. (1175° F./1225° F.) for at least one hourat temperature. In a second preferred embodiment, a steel strip producthaving an ultimate tensile strength of greater than 900 MPa and totalelongation of at least 30% can be produced from strip consistingessentially in weight percent of C 0.09/0.11, Mn 7.0/7.2, Si less than0.5, Al less than 0.1, balance Fe and incidental impurities. To achievethese properties the hot roll finishing temperature should be within therange of 871° C./982° C. (1600° F./1800° F.), with a coiling temperaturewithin the range of 566° C./621° C. (1050° F./1150° F.), and a boxannealing temperature within the range of 635° C./663° C. (1175°F./1225° F.) for at least one hour at temperature.

While one or more preferred embodiments of the invention have beenidentified, other configurations and modifications can be providedwithin the scope of the present invention.

1. A method for producing a high strength, high ductility steel strip,said method comprising: (a) providing a steel slab consistingessentially of in weight percent carbon 0.05/0.20, manganese 3.0/8.0,silicon less than 0.5, aluminum less than 0.1, the balance iron andincidental impurities; (b) hot rolling the steel slab to strip with afinish rolling temperature above Ar1+50° C.; (c) cooling the hot rolledsteel strip at a rate sufficient to form a microstructure that consistsof greater than 50 percent by volume of martensite; (d) then boxannealing the steel strip in coil form for a minimum of one hour at atemperature within a range of Ac1 to Ac1+50° C.; and (e) cooling thesteel to room temperature.
 2. The method of claim 1, wherein in thecooling step, the hot rolled steel strip is cooled at a rate sufficientto form a microstructure that consists of greater than 90 percent byvolume of martensite;
 3. The method of claim 1, wherein the boxannealing step is carried out at a temperature within the range of 593°C. to 654° C.
 4. The method of claim 1, wherein the annealing cycle timeis controlled so that the maximum difference in temperature between anouter lap of the coil and an inner lap of the coil is less than 28° C.during the anneal cycle.
 5. The method of claim 1, wherein the hotrolled strip is coiled at a temperature within the range of 566° C. to621° C.
 6. The method of claim 1, wherein the temperature in the boxannealing step is within the range of 635° C. to 663° C.
 7. The methodof claim 1, wherein the hot roll finishing temperature is within therange of 871° C. to 982° C., the hot rolled strip is coiled at atemperature within the range of 566° C. to 621° C., and the temperaturein the box annealing step is within the range of 635° C. to 663° C. 8.The method of claim 7, wherein the steel slab consists essentially of C0.09/0.11, Mn 5.7/6.0, Si less than 0.5, and Al less than 0.1, thebalance iron and incidental impurities.
 9. The method of claim 7,wherein the steel slab consists essentially of C 0.09/0.11, Mn 7.0/7.2,Si less than 0.5, Al less than 0.1, the balance iron and incidentalimpurities.
 10. A hot rolled and box annealed steel strip producedaccording to the method of claim 1, said steel strip having an ultimatetensile strength of at least 800 MPa and total elongation of at least25%.
 11. A hot rolled and box annealed steel strip produced according tothe method of claim 8, said steel strip has an ultimate tensile strengthof at least 800 MPa and a total elongation of at least 35%.
 12. A hotrolled and box annealed steel strip produced according to the method ofclaim 9, said steel strip has an ultimate tensil strength of at least900 MPa and a total elongation of at least 30%.
 13. A method forproducing a high strength, high ductility steel strip, said methodcomprising: (a) providing a steel slab consisting essentially of inweight percent carbon 0.05/0.20, manganese 2.0/8.0, silicon less than0.5, and aluminum less than 2.0, the balance iron and incidentalimpurities; (b) hot rolling the steel slab to strip with a finishrolling temperature above Ar1+50° C.; (c) cooling the hot rolled steelstrip at a rate sufficient to form a microstructure that consists ofgreater than 50 percent by volume of martensite; (d) then box annealingthe steel strip in coil form for a minimum of one hour at a temperaturewithin a range of Ac1 to Ac1+50° C.; and (e) cooling the steel to roomtemperature.